Complete silence is found only in laboratories called anechoic rooms. The walls and ceilings, made of blocks of special sound-sucking materials, are more than three feet thick, while floor coverings are six-foot layers of feathers or cotton wool. Silence here can be as painful to the ears as the din(continuous loud noise)of a steelworks or a rocket blast-off, yet scientists get used to this and stay in these silent rooms for hours at a time, using microphones and electronic equipment to test the various materials being developed to make the world a less noisy place. Architects have used scientific discoveries to solve noise problems in a number of ways. Walls are hollowed(having empty space inside)and then filled with sound-sucking materials similar to cotton wool. Extra-thick carpets cover the floors, and thick woolen curtains cover the windows. Air conditioning and heating channels are made less noisy by sound-sucking materials. Unfortunately, these techniques and others often work too well in some buildings. Noise-proof rooms become almost anechoic and people living in them are disturbed by the lack of sound. One way of handling this problem is to use what they call " sound perfume" — artificial(similarly produced, made by man)noise is piped to rooms through small loudspeakers.